The concept of Orthogonal Sub Channels (OSC) proposed in “Voice capacity evolution with orthogonal sub channel, see” 3GPP TSG GERAN Meeting #33, GP-070214 has been well accepted. One reason is that the dramatic growth of the subscriber base in developing countries imposes a tremendous pressure on the Base Transceiver Station (BTS) hardware resources. Therefore, a study item for a technique dubbed MUROS (Multiple User Reusing One Slot) has been opened in the standardization of GSM, see “New Study Item on Multi-User Reusing One Slot (MUROS” 3GPP TSG GERAN Meeting #36, GP072027).
OSC is a multiplexing technique that allows two users to share the same frequency and time slot. It relies on Quadrature Phase Shift Keying (QPSK) modulation in the downlink channel. The I and Q branches of a modulated signal form two sub channels. The data carried by the I branch belongs to a first user, while the data carried by the Q branch belongs to a second user. Orthogonality is preserved by using a root raised cosine pulse shaping filter with a bandwidth equal to the reciprocal of the symbol period, although other transmit pulses may also be employed. At the receiver side, the mobile stations (MS) rely on orthogonal training sequences in order to separate the sub channels, see 3GPP TSG GERAN Meeting #33, GP-070214. In the uplink channel, the two mobile stations sharing the same channel also transmit in the same frequency and time slot. The base station separates the two users using a multi-user detector, e.g. successive interference cancellation.
It has been stated in “New Study Item on Multi-User Reusing One Slot (MUROS” 3GPP TSG GERAN Meeting #36, GP072027, that the physical layer for MUROS must support legacy mobile stations in one of the sub channels. However, it has been reported, see e.g. “The Performance of OSC and Feasibility Analysis”, 3GPP TSG GERAN Meeting #36, GP071663 and “Discussion Paper on OSC”, 3GPP TSG GERAN Meeting #36, GP071785, that OSC may not be backward compatible with legacy Gaussian minimum shift keying (GMSK) mobile stations. A problem arises in the downlink channel, because a legacy receiver exhibits very poor performance when the transmitted signal is Quadrature phase-shift keying (QPSK) modulated.
Moreover, some concerns have been raised about the constraints that OSC imposes on power control and the need to subdivide the cells of a cellular radio system, leading to additional handovers and hence presenting a potential to degrade some of the Key Performance Indicators (KPI) such as dropped calls, see also “On Orthogonal Sub channels”, 3GPP TSG GERAN Meeting #36, GP071720. Also so far, none of the proposals for MUROS as set out in Voice capacity evolution with orthogonal sub channel,” 3GPP TSG GERAN Meeting #33, GP-070214 and “Speech capacity enhancements using DARP”, 3GPP TSG GERAN Meeting #36, GP071739 fulfills the backward compatibility requirement for legacy mobiles.
Hence, there exist a need for a method and a system that eliminates or at least reduces the negative aspects of introducing OSC in existing cellular radio systems. Also, there exists a need for an improved power control method for MUROS.